Electric heater assemblies wherein a support and mounting assembly supports a plurality of insulators which in turn support a coiled resistive electric heater element are shown in U.S. Pat. Nos. 5,329,098 (Howard et al.), 5,324,919 (Howard et al.) and 4,617,547 (Howard et al.). In these patents, there are several types of heater support and mounting assemblies shown supporting an electrically continuous heater element, referred to hereinafter as a single-stage heater. The single-stage heater assembly in U.S. Pat. Nos. 5,329,098 and 5,324,919, which are hereby incorporated by reference, show each end of the heater element extending to a terminal block with two terminals mounted on a side wall toward one end of the housing. Generally, these single-stage heaters comprise a support element, for holding and supporting the insulators which in turn support the heater element within a housing. The housing is generally a continuous U-shaped sheet of material designed to be inserted into a specifically sized open portion of an air duct, for instance, in an appliance such as an electric clothes dryer.
The compact design of appliances where heaters of this type are typically employed limits the amount of space available for insertion of an electric heater assembly. In addition, the appliance manufacturers typically offer several models of any particular appliance which incorporate a variety of different options. The various models are ideally designed to simplify manufacturing by utilizing interchangeable parts, which provide different features, that are assembled using the same equipment and tooling. It has long been recognized that the incorporation of a heater assembly having an ability to vary the heat output and/or provide better heating characteristics into an appliance would enable the appliance manufacturer to offer all or some models of an appliance with additional features making them more marketable. However, this new feature would not be valuable to the appliance manufacturer unless the new heater assembly was compatible with the standard single-stage heater assembly such that it could be readily installed using the same equipment and tooling on appliance models designed to accommodate the standard heater assembly.
Other means of controlling heat output by varying the electric power to the standard single-stage heater assemblies have been attempted but they all involved complex control circuitry and/or other heat producing elements, such as rheostats. These were considered impractical or otherwise undesirable because they were either too costly or involved additional design measures to address the additional heat generated. Therefore, there is a need for a heater assembly that provides multi-level heating within a housing that is readily interchangeable with standard single-stage heater housings that can be controlled by conventional power switching circuits.